During the machining of valve seats, particularly on heat engines, a maximum precision is sought for the seats. The seats are machined on a lathe, the shaping tool used being guided over a spindle, which is itself guided by a guide rod in the valve guide of the valve seat to be machined. The precision of the valve seat which can be obtained by this machining directly depends on the fine adjustment or on the position of the guide rod in the valve guide. Even if this adjustment is virtually perfect, it is impossible to avoid play between the guide rod and the valve guide, even if this play is small. This is particularly the case when the valve guide is slightly worn and therefore its diameter is enlarged towards its ends. The present invention relates to a method and device allowing the optimum centering of such a guide rod in a valve guide by being placed in the center of the play existing between the guide rod and the valve guide.
Conventional devices for machining valve seats are constituted by a magnetic column and a pivoting arm provided with a spindle which is fixed to this column. The magnetic column has a magnetic base which can be placed directly on the cylinder head or on a fixing plate and can be fixed in position on it. The magnetic column is constituted by a magnetic base with a tubular column onto which the pivoting arm can be locked with the possibility of height adjustment. The pivoting arm carries at its end a support and a clamping device for a pivoting ball, in which is mounted the spindle provided with the guide rod. The spindle can thus be oriented in all directions and locked in any pivoting position by means of the clamping device. The shaping or cutting tool is then mounted on the spindle. The rotation of the spindle can be carried out by hand, but also by means of mechanical, pneumatic, hydraulic or electrical drives. The device is used according to a method in which the guide rod, in the unclamped state of the pivoting ball and of the pivoting arm, is inserted into the valve guide. A guide rod is used which has the closest possible fit and which must have the smallest possible play in the valve guide. After the insertion of the guide rod, the magnetic field is actuated so that the magnetic base is fixed in a rigid manner. After having fixed the pivoting arm in position and having locked the ball, it is then possible to machine the valve seat. The rotary shaping tool is lowered progressively onto the valve seat and material is removed until the seat is cleanly machined. It will be understood that with this method the guide rod inevitably has a certain play in the valve guide and that within this play it has a certain imprecision with regard to its exact central position. If, in addition, the valve guide is a little worn, this play increases. Even if it is a matter of only fractions of degrees of angle, such an imprecision in the position of the guide rod, which must serve as a guide for the turning tool, has a considerable influence on the subsequent fine fitting of the valve in the machined seat. In order to overcome this, certain devices are equipped in particular with a ball pivoting on a spherical air cushion such that, during the insertion of the guide rod, in particular the friction forces between the ball and the clamping device are minimized.